Generally, injection molds have a steel body because of a number of advantages such as high durability and high repeated transfer accuracy. However, fabrication of a steel mold is difficult and takes a long time to invite a considerable increase in fabrication cost. Besides, steel molds have a number of disadvantages such as low heat conductivity and heaviness. Taking these points into consideration, injection molds of a material other than steel have been developed and put in use. Among the new developments is an injection mold of aluminum which is superior in machinability and can be fabricated in a shorter time and at a lower cost. Besides, high heat conductivity can contribute to shorten the time of a molding cycle. Further, an aluminum mold which is lighter in weight is advantageous from a standpoint of facilitating various operations such as connections and disconnections to and from a molding machine, mold transfer and mold assembling.
Therefore, aluminum, especially an aluminum alloy is hopeful as a material for injection molds. However, as compared with an iron mold, an aluminum mold is inferior in hardness and resistance to abrasion. Plastics which are used in injection molding are mostly thermoplastic resins, and in many cases a reinforcing material such as glass fiber, for example, is mixed into a plastic material for the purpose of enhancing the strength of moldings. Aside from those cases where glass fiber is mixed only in a small proportion, when molten plastic material is introduced into a mold cavity through a sprue, runner and gate, it is very likely that wall surfaces en route are abraded to a considerable degree particularly in a case the plastic material contains glass fiber in a high proportion, for example, in a proportion in excess of 40%. This gives rise to a problem that abraded aluminum powder is mixed into the molten plastic material to cause abrasion of inner surfaces of a mold cavity, degrading transfer accuracy of the mold and surface accuracy of moldings in a prematurely early stage, resulting in a shortened life span of the mold. For these reasons, despite a number of advantages, aluminum molds have been applied to injection molding on a small scale, producing up to 100 moldings from an aluminum mold, and have not been resorted to in mass scale injection molding.
Various attempts have thus far been made to prolong the service life of aluminum molds. For example, in one case, an attempt is made to fabricate an injection mold by the use of a hard aluminum alloy which is less susceptible to abrasion like extra super duralumin (A7075), for example. However, as compared with steel molds, even extra super duralumin is still unsatisfactory in durability and found to be insufficient in hardness as an injection mold depending upon the nature of a plastic material to be molded.
On the other hand, in another case, an attempt is made to prolong the service life of an aluminum mold by hardening interior surfaces of a mold including a mold cavity. For example, Japanese Laid-Open Patent Application H7-3470 describes fabrication of a mold assembly by the use of an aluminum alloy, the mold assembly consisting of stationary mold and a movable mold which internally define a cavity. In this case, with or without a prior alumite treatment, an ion beam, mainly of argon, is irradiated on mold surfaces to form a hardened layer thereon, followed by deposition of a nitride film layer by sputtering in a nitrogen atmosphere using titanium and aluminum as targets and further followed by electroless nickel plating.
In order to impart aluminum mold surfaces with a certain degree of hardness, the hardening treatment process requires multiple stages of hardening treatments as mentioned above, which are extremely complicate and troublesome, depriving of inherent advantages of an aluminum mold such as high machinability, shorter fabrication time and low cost. In addition, the electroless nickel plating in the last stage is effected on the entire surfaces of a mold. However, in case entire joint surfaces on both of stationary and movable mold members are plated with nickel which is lower in heat conductivity than aluminum, the stationary and movable mold members are put in a thermally shielded state, resulting in degradations in heating efficiency and in cooling efficiency as well. This may give rise to heat variations within the mold cavity due to a temperature difference between stationary and movable mold members to invite degradations in molding accuracy and mold releasing failures.